Heat-sensitive recording papers are known which comprise a heat-sensitive layer containing microcapsules containing, as core materials, a component capable of undergoing a color formation reaction (color former) and an organic solvent, and another component capable of reacting with the color former to develop a color (color developer). The latter being exterior the microcapsules, as described, e.g., in Japanese Patent Application (OPI) No. 190886/84 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") and Japanese Patent Application No. 6493/85.
These recording materials are not of the type where a reactive substance contained in microcapsules and a reactive substance outside of the microcapsules are brought into contact upon rupture of the microcapsules by heat or pressure to thereby develop color as in conventional recording materials, rather, are of the type where a reactive substance present inside or outside of the microcapsules is allowed to permeate through the microcapsule wall by application of heat to thereby cause color formation reaction therebetween.
In more detail, the capsule wall changes from a glassy state into a rubbery state upon instantaneous heating with a thermal head, and the color forming component is diffused through the wall to contact with the another component to cause the color forming reaction.
If a thermal head contacts the microcapsule wall softened by heating, the recording material will stick to the thermal head, resulting in poor running properties. With the recent development of rapid print out with a heating element, the demand has arisen to improve the handling and running properties of heat-sensitive recording sheets.
Good handling properties means the ability to be free from abrasion fog due to scratches on handling, etc. which causes stains on the recording surface. Good running properties means the ability to be free from "head stain" due to adhesion of the recording layer to a thermal head during continuous running on a facsimile machine, a printer, etc., to be free from a reduction of image quality arising from head stain, or to be free from the generation of "stick noise" due to adhesion between the thermal head and the recording paper during running, and the like.